Current harvesting machines have been developed for meeting the increasing needs of capacity and output under a wide range of field and crop conditions. In modern high capacity combine harvesters, one of the critical factors is the capacity of the cleaning means, comprising grain pans and sieves for receiving and cleaning the crop material from a threshing and separating mechanism. Besides grain kernels, this threshed crop material comprises discardable material, such as chaff and straw particles, which is blown out of the combine harvester by a ventilator or fan. In order to obtain an effective cleaning action, the fan has to generate a forceful and even air flow over and through the complete width of the sieves.
To that end, a conventional centrifugal fan structure, which comprises a fan, mounted in a generally volute-shaped housing with central air inlets in the side walls and a main air outlet duct directed towards the sieves, has been provided with an secondary air inlet in the top wall of the housing. Such configuration has been disclosed in GB-A-2,063,034. In order to keep impurities, such as crop stems, out of the fan housing, the secondary air inlet is provided with a perforated shield. For an effective air flow, deflector plates can be mounted in the air duct between the fan and the sieves, but no other obstructions may be placed therein. The air flow is directed via a main outlet duct towards an upper chaffer sieve and a lower cleaning sieve and via an additional outlet duct towards a pre-cleaning sieve and an associated grain pan. Said sieves and grain pan extend rearwardly and slightly upwardly to the rear of the harvester.
It has been experienced that under some circumstances grain kernels can move towards the forward end of the sieves and the associated grain pan and fall into one of the outlet ducts. This happens when their front portion is fully loaded with crop material and the combine harvester is suddenly halted and/or reversed. The crop material may also slide into the outlet ducts during downhill travel of the combine harvester. The air flow from the fan may blow back the lighter portion thereof towards the sieves, but this effect quickly diminishes when the sieves are fully loaded and the air flow is choked consequently. Even during normal operation of the harvester, crop particles may roll to and over the forward edge of the sieves: this occurs when the front portion of the sieves or the associated grain pan is polluted with a layer of sticky crop material. The even surface thereof gets no grip on the newly arriving crop particles, which migrate downwardly and forwardly until they fall into one of the outlet ducts.
The heavier portion of the crop material, including the grain kernels can penetrate further into the outlet ducts, until it is engaged by the blades of the rotating fan. Most of the grain kernels are projected one or several times against the walls of the fan housing and are finally launched into the main outlet trough and the clean grain auger duct below the lower cleaning sieve, from where they are conveyed to the grain tank. A considerable amount of kernels, however, is thrown upwardly and through the holes in the perforated plate of the secondary air inlet. They are scattered around the fan housing and fall on the field or accumulate on top of near harvester parts, such as the traction gearbox and the traction beam.
These grain losses are easily spotted and, although their cause may be incidental, e.g. a sudden halt of the machine, the losses should be avoided to the maximum extent. When they result from harvesting operations during prolonged downhill travel, one could consider to reduce the travelling speed of the harvesting machine in order to diminish the load on the sieves and hence the tendency of the crop material to slide to the leading edges of the sieves and the grain pan, but the consequent capacity losses largely exceed the value of the grain lost through the secondary air inlet.
The grain losses could also be reduced by reducing the diameter of apertures in the perforated plate, but this will have a choking effect on the air flow through this inlet and hence affects the efficiency of the fan structure.